Cellulose acetate fibers are widely used as filters for various tobacco products, e.g., cigarettes. The fibers themselves are often referred to as "filter tow" and are generally produced by forcing an acetone/cellulose acetate dope through a spinnerette under pressure sufficient to form a continuous bundle of fiber, which is subsequently compacted to form a filter rod.
One of the primary methods for determining the filtration performance range for a given sample of filter tow is the calculation of the so-called "capability curve". A capability curve's minimum point can be consistently determined if care is taken by the plugmaker operator to obtain filter rod recess which is indicative of a minimum point. A plugmaker operator can often repeatedly adjust to the same amount of recess which he feels represents a minimum point. However, a different operator may get a significantly different amount of recess which he felt represented the same minimum point and neither one may repeatedly have obtained the desired amount of recess. While apparent recess differences may appear to be small, the pressure drop of the filter rods can be markedly different.
Specific properties of filter rods are affected by the properties of the tow used to make the rods and by other rod properties.
A capability range of filter rod weight and pressure drop is possible with cellulose acetate filter tow because crimped tow is extensible. The relationship between the weight of tow in a filter rod and the pressure drop of the rod is direct and linear for a given tow sample. As the speed of the tow processing unit is increased or decreased in relation to the speed of the plugmaker, there will be a corresponding increase in rod pressure drop.
To determine the capability range of a given tow item on a specific filtermaking machine, filter rods should be produced at the highest tow weight possible (i.e., maximum capability) and the lowest weight possible (minimum capability). The maximum capability point is normally defined as the point at which the garniture of the plugmaker will not accept additional tow. Attempts to increase the tow weight beyond this point will result in roll wraps, failure to seal the rod seam, or excessive variation in filter rod properties. The minimum capability point is normally defined as supplying the lowest tow weight possible to the garniture while maintaining an acceptable rod from the standpoint of no wrinkles and very slight end recess (approximately 1.0 mm). In capability evaluations, rods at midpoint weight (halfway between minimum and maximum) are also produced. The rods produced at these three conditions (maximum, minimum, and midpoint) are tested from circumference, weight, and pressure drop and the results are plotted graphically. When a line is drawn through these plotted points, the pressure drop/weight relationship can be determined at any point along the line. The line generated by this procedure is generally referred to as a "capability curve", see FIG. 4.
An important point to remember regarding the "capability" of a tow item on a particular type of filter making equipment concerns target placement. If the target is not well placed on the capability curve, it is likely that processing difficulties or poor quality rods may result. If the above parameters are used for the determination of the minimum capability point (rod recess approximately 1.0 mm and no wrinkles), selection of a target pressure drop of at least 15 percent above the minimum pressure drop is preferred. If slow speed filter making equipment is used or if standard tow transport jet systems are used on high speed filter makers, the target pressure drop should not be more than 25 percent above the minimum pressure drop. If "capability extension" systems such as the EASTMAN Type II tow transport jet or the modified HAUNI tow transport jet system are used, the upper limit for target pressure drop may be increased to as much as 50 percent above the minimum pressure drop. In this fashion, target pressure drop is referenced to the minimum point.